The invention herein described pertains generally to determining the position of a marine vessel, and more specifically to adaptive means for processing signals indicative of the deviation from vertical of a marine riser to produce an estimate of horizontal vessel position.
There is increasing interest in exploring for oil and minerals on or beneath the ocean beds in deep water, producing oil and minerals from such locations, conducting marine research in very deep water and maintaining and servicing equipment used in connection with the foregoing activities. Such activities require the ability to rapidly and accurately ascertain marine vessel position relative to an underwater location of interest. High quality position information is also demanded in other specific applications in which it is required to maintain fixed position of a floating marine vessel.
A variety of schemes have been developed for determining marine vessel position. Acoustic systems have been increasingly used because they do not require physical connection to the ocean bed or object of interest, and because of their potential capability of producing accurate position information in very deep water. However, the quality of position indications produced by acoustic systems is generally dependent on acoustic paths having constant transmission characteristics. The subsea environment typically does not provide ideal acoustic transmission paths. Factors resulting in variable transmission characteristics include reflection and/or refraction of signals from thermal layers in the water, scattering of signals from water borne particles and reflection of signals from underwater structures. These factors may result in spurious indications of vessel position or complete loss of position information for short periods of time. In addition, acoustic signals emanating from sources other than the position indicating system may cause spurious position determinations. These factors produce a severe acoustic signal transmission environment having effects on signal transmission which are not presently susceptible of complete analysis. The uncertainties in operation of an acoustic position indicating system are not acceptable in the most critical vessel positioning applications. Accordingly, it may be desired or required to have an alternative or backup system other than an acoustic system. Obviously, in critical applications such an alternative or backup system also must be capable of reliably and accurately indicating vessel position.
One alternative system and technique which has been used to determine the horizontal position of a marine vessel relative to an underwater wellhead is to sense the deviation from vertical of a marine riser, oil well drill string, or cable extending from the vessel to the wellhead or a location near the wellhead. For simplicity the remainder of the discussion will refer only to a riser, with the understanding that the system or technique may actually involve a drill string, cable or other flexible member suspended from the vessel. A signal indicative of the angular deviation may be used in a system to display and/or control vessel position. Angular deviation of the riser from vertical typically has been sensed by a sensor located near either the upper or the lower end of the riser. Such a scheme is satisfactory provided that the riser acts in a predictable fashion. Riser dynamics are reasonably predictable for shallow water depths. However, as the water depth increases, the measured angles become increasingly less a direct indicator of vessel position.
U.S. Pat. No. 4,205,379 issued in the name of M. Fox, et al discloses a version of a riser angle reference system which offers some improvement over conventional systems. It includes sensors for sensing the instantaneous vertical slope angles of the riser at both its upper and lower ends. The angle signals are filtered to remove higher order frequencies and combined in accordance with a position equation which compensates for phase lag at the lower sensor. Although such an approach is superior to one which utilizes a single riser angle, it is limited in capability to producing a position estimate based on only one assumed riser response for each pair of measured angles at any given depth. Such an assumption is not valid in many instances.
A long riser acts in a very flexible manner and deviates substantially from a straight line between the vessel and the wellhead. It has various response modes depending on excitation stimuli, riser parameters and other factors. The various modes result in different riser shapes, e.g., an s shape. The first several lower order modes fall within the response range of a typical dynamic positioning system (i.e., a positioning system which utilizes thrusters to maintain vessel position). Accordingly, the various modes must be correctly accommodated to achieve satisfactory position control.
Riser excitation stimuli include currents which differ substantially at different depths, and wind and wave action. There is a substantial delay between the time that vessel motion occurs and the time at which corresponding motion appears at the lower end of the riser. Angular deviation at both ends of the riser is contaminated with higher frequency responses to wind and waves which are typically alternating in nature. Particularly in connection with wave action, the period of vessel movement is relatively short and the vessel is continuously driven a short distance either side of and then returned to its desired position. With reference to vessel position control, significant energy would be required to attempt to oppose this action. Further, such operation is not required in most applications and would be grossly inefficient in terms of equipment operation and fuel consumption.
It has been found that the overall quality of position indication of a riser angle reference system can be much improved by detecting angular deviations at both the upper and lower ends of the riser and filtering the signals indicative of the angles in an adaptive filter whose coefficients are adjusted in accordance with a best fit mathematical model of the riser selected from a plurality of riser models. Accordingly, the useful position information reflected in the angles can be extracted for any important riser mode while avoiding the complications caused by lag at the lower end of the riser and extraneous high frequency components at the upper end of the riser.